1. Field of the Invention
This invention relates to apparatus and methods for computing the position of a mobile device by use of wireless positioning signals, such as GPS systems.
2. Description of Related Art
Position location devices are becoming increasingly popular, not only for ships at sea or adventurers in the backcountry, but also for anyone who uses a cell phone in daily life. The increasing number of cell phones, coupled with the popularity of personal position location devices, has encouraged development of rapid, high sensitivity methods for acquiring the signals used to determine position.
Position location technologies typically utilize wireless positioning signals concurrently transmitted from known locations. In GPS systems, these positioning signals are concurrently transmitted from a multiplicity of satellites at a known time, with a predefined frequency. On the ground, a GPS receiver acquires a positioning signal from each satellite within its view of the sky. The times of arrival of the positioning signals along with the exact location of the in-view satellites and the exact times the signals were transmitted from each satellite are used to triangulate the position of the GPS receiver.
Positioning signals, and particularly GPS signals, include high rate repetitive signals, or “codes”, called pseudorandom (PN) sequences. The codes available for civilian applications are called C/A codes, and have a binary phase-reversal rate, or “chipping” rate, of 1.023 MHz and a repetition period of 1023 chips for a code period of 1 msec. The pseudorandom sequences in the GPS system belong to a family known as “Gold codes”. Each GPS satellite simultaneously broadcasts a signal at a carrier frequency with its unique Gold code.
At a receiver, the electromagnetic energy at the carrier frequency is observed, and this observed energy is processed to search for the possible presence of signals from any GPS satellites that may be in view. At the time of observation by the receiver, the particular GPS code and the phase delay are not known. The object of the receiver is to identify the GPS code(s) in the observed energy about the carrier frequency, and determine the phase delay of each identified GPS code. However, because the GPS code and phase delay is initially unknown, an approach is typically employed in which a first GPS code is hypothesized and a number of phase assumptions are then tested sequentially until the GPS signal has been either identified or determined not to be present. The process is then repeated for each other GPS satellite that may be in view.
Receiving positioning signals from GPS satellites can be difficult due to a number of factors. For example, GPS signals are transmitted at relatively low power, and from a great distance. By the time the GPS signals travel from earth orbit to a receiver, their initially low power has been greatly reduced, rendering the signal extremely weak at the receiver.
Another problem relates to frequency errors that can affect one or more of the positioning signals. For example, the carrier frequency may shift slightly over time due to Doppler effects. In the receiver, the oscillators and other electronic devices that receive and process the signal can introduce errors such as slight shifts in frequency, which can complicate reception. If the frequency shift is constant, a Fourier transform (e.g., an FFT) approach can be used; however, further complications result when this frequency shift varies over time; i.e., when the frequency shift is not constant over the observation time (the data block). In order to address the problem of time-varying frequency errors, the length of the data block for coherent processing (coherence length) is usually limited to a fraction of a second (e.g., 20 milliseconds); otherwise, the frequency errors could greatly degrade the system sensitivity. In order to increase the system sensitivity in the possible presence of frequency shifts, a number of successive coherent processing operations may be done for a number of time periods (e.g., five to twenty), and the results are added together non-coherently to provide an indication of the signal over periods of one second or more. It would be a significant advantage if there were a system available that could perform a single coherent processing operation over longer periods of time; i.e., if the coherent integration length could be significantly increased.